It's not? In the film they say that the Federation public doesn't know about it, and that not even the Council really knows what's going on, and Picard only gets to know what the "anomalies" in the Briar Patch are because he gets the Enterprise there uninvited. The fountain of youth effect of the metaphasic radiation was something the Enterprise crew found out for themselves, they weren't told about it.

nor are the Son'a members of Starfleet.

The Son'a are a group of shady individuals that supply the Dominion with Ketracell white, used illegal weapons, and only hopped in bed with the Federation because the planet was in their territory. The Son'a certainly kept it a secret, as they also told Starfleet only what they needed to know.

Why would they and Starfleet officers agree to cover-up in order to protect the Baku's selfish lifestyle?

Because they probably don't see it as "selfish".

yes, the REMOVAL of the Baku was kept secret for obvious reasons, but the resource itself COULDN'T have stayed secret, because they eventually planned to have the Federation population as a whole use it. ( even if they planned to put the magic particles in the drinking water secretly or something like that, don't you think that when blind folks started re-growing eyes and boobs started to magically firm-up, questions would be asked?)